1. Field of the Invention
The present invention relates to a cam driving mechanism for driving a cam, a belt transporting apparatus provided therewith for rotating an endless belt such as a transport belt or an intermediary transfer belt, and an image forming apparatus provided therewith.
2. Description of Related Art
Various types of image forming apparatuses have been conventionally proposed. In one type of image forming apparatus, there are provided an endless transport belt rotated in a predetermined direction and an image forming section arranged along the transport belt, and a toner image formed by the image forming section is transferred onto a recording medium transported by the transport belt. In another type of image forming apparatus, one toner image after another are sequentially formed on an endless intermediary transfer belt by a plurality of image forming sections and then these toner images are transferred onto a recording medium all at once.
In image forming apparatuses that employ an endless transport or intermediary transfer belt as mentioned above, improper arrangement, that is, so-called misalignment, of a belted roller such as a drive roller or a tension roller ascribable to strain in the apparatus body may lead to meandering or lopsiding of a transport belt. Meandering or lopsiding of the transport belt causes the transported paper to be displaced sideways (left or right), and thus makes displacement of the transferred image more likely.
In the case of tandem-type color image forming apparatuses which form a color image by operating a plurality of image forming sections, meandering or lopsiding of the transport belt degrades the positioning of the images of different colors formed by the individual image forming sections relative to one another, and thus makes color displacement among the toner images of the different colors more likely. Also in image forming apparatuses of the intermediary transfer type, meandering of the intermediary transfer belt causes similar problems.
In conventional belt transporting apparatuses, meandering or lopsiding of a belt is detected by a sensor, and according to the amount of meandering or lopsiding, the alignment of one or more belted rollers (their angle in the obverse/reverse direction of the belt) is adjusted, so that the meandering or lopsiding is automatically corrected. And a common method of adjusting the alignment of a belted roller is changing the rotation angle of an eccentric cam or moving distance of a linear motion cam by using a pinion and rack mechanism, which swings one end of the rotary shaft of the belted roller.
For accurate alignment adjustment using a cam, for example, according to one known method, the amount of driving of the motor for rotating or moving a cam is changed in accordance with the amount of displacement of the cam per unit angle or distance; according to another known method, a cam is used that is so shaped that the meandering speed of the belt is increased or decreased at a predetermined rate according to the rotation angle so as to constantly keep equal the rotation angle or moving distance of the cam and the amount of meandering corrected.
In a case where alignment adjustment is achieved by changing the rotation angle or moving distance of the cam as described above, the amount of driving of the motor for rotating or moving the cam needs to be controlled accurately. For that purpose, according to a known method, a home position flag is provided at the end of the output shaft of a steering motor opposite from the end thereof at which the cam is provided, and by detecting the phase of the flag by a home position sensor, the phase of the rotation cam is detected; in addition, based on information on the output voltage of a belt edge sensor, the number of drive pulses for the steering motor is determined; thus the rotation angle or moving distance of the cam is controlled
In a case where alignment adjustment is achieved by using a stepping motor as the motor for rotating the eccentric cam or moving the linear motion cam, the position (rotation angle or moving distance) of the cam can be controlled according to the number of drive pulses transmitted to the motor; however, in a case where a brush motor is used as a cheap alternative to a stepping motor, which is expensive, controlling the amount of driving of the motor requires position detecting means for detecting the position (rotation angle or moving distance) of the cam and home position detecting means for detecting the reference position (home position) relative to which the cam is rotated.
Here, attempting to detect both the cam position and the reference position with a single member (pulse disk) inconveniently leads to the pulse disk having low resolution and hence low detection accuracy, making accurate adjustment of roller alignment impossible. On the other hand, detecting them with separate members requires a large space for arrangement of detection-related components as well as two separate sensors for detection of the cam position and the home position, disadvantageously for size reduction and cost reduction of the apparatus.
Although the foregoing discusses, as an example, a driving mechanism for a cam used for alignment adjustment of a belted roller on which an endless belt is wound, similar problems have also been encountered with other types of cam driving mechanism that adjust the amount of displacement of a member on the basis of the phase of a cam.